1. Field of the Invention
The present invention relates to an apparatus for measuring the remaining amount of bobbin thread wound on a bobbin accommodated in a shuttle of a sewing machine.
2. Related Art Statement
Japanese Unexamined Utility Model Application laid open under Publication No. 61(1986)-180685 discloses an example of the remaining bobbin-thread amount measuring apparatus. The disclosed apparatus is of optical type wherein, when the rotation of a rotating hook of a shuttle is stopped, a light is emitted toward the outer surface of a roll of bobbin thread wound on a bobbin accommodated in the rotating hook and the light reflected from the roll of bobbin thread and detected by an optical sensor is utilized for determining the amount of the bobbin thread remaining on the bobbin. However, the sensitivity of the optical sensor is lowered due to dust or oil adhered thereto. Another problem with the prior apparatus is that the sensitivity of the optical sensor changes depending upon colors of bobbin thread. Thus, in some cases, the optical-type apparatus cannot accurately measure the remaining amount of bobbin thread.
Meanwhile, Japanese Examined Patent Application laid open for opposition purpose under Publication No. 61(1986)-43075 discloses a near-end detecting apparatus. The disclosed apparatus is of mechanical type which includes (a) a measurement bar which is displaceable along a displacement path from outside a shuttle toward the axis line of a bobbin accommodated in the shuttle, (b) a meaurement-bar driver which displaces the measurement bar along the displacement path so that a free end of the bar contacts the outer surface of a roll of bobbin thread remaining on the bobbin, after the rotation of the shuttle has been stopped, the driver retracting the measurement bar away from the outer limit of a locus of rotation of the shuttle before the rotation of the shuttle is re-started, and (c) a near-end microswitch which is adapted to close when the displacement amount of the measurement bar necessary for the bar to contact the bobbin thread exceeds a predetermined value while the bobbin thread is gradually consumed, the closing of the microswitch triggering an alarm. Thus, the near-end detecting apparatus identifies with accuracy that the remaining bobbin-thread amount has been reduced to near-end, i.e., near-zero.
However, in the near-end detecting apparatus, the measurement-bar driver continues to drive the measurement bar till the bar contacts the bobbin thread on the bobbin. Consequently, the measurement bar collides with the bobbin thread or the bobbin at an excessively high speed. Therefore, as the remaining bobbin-thread amount decreases, the bobbin thread more frequently breaks or damages because of being nipped between the bobbin and the moving bar.